Background
Adaptive response (AR) occurs when cells pre-exposed to an extremely small adaptive dose (AD) of a DNA damaging agent develop increased resistance to a subsequent exposure to a higher challenge dose. Past studies have shown that pre-exposure of cells to non-ionising radiofrequency fields (RF) is capable of inducing an AR however, more investigation is required.

Objective
The objective of this study was to investigate the influence of the cell cycle on the AR induced by the exposure of human blood lymphocytes to RF.

Methods
Human peripheral blood lymphocytes in the G0-, G1- or S-phase of the cell cycle obtained from the peripheral blood of nine healthy non-smoking male donors between the ages of 20 and 47 years, were exposed to an AD of 900 MHz RF at an average specific absorption rate of 1.25 W/kg for 20 hours and then treated with a challenge dose of 100 ng/ml mitomycin C (MMC). Wire patch cells were used to expose the cells. Un-exposed and sham-exposed controls as well as cells treated with MMC alone were included in the study. The incidence of micronuclei was evaluated on coded slides examined at 1000x magnification to determine the induction of an AR.

Results
In basic cultures, compared with untreated controls, treatment of the lymphocytes with 1 ng/ml MMC resulted in a slight increase in micronuclei frequencies with no significant effect on the percentage of binucleate cells or proliferation. In experimental cultures, lymphocytes which were pre-exposed for 20 h to 900 MHz RF in G0-, G1- and S-phase of cell cycle did not show significant changes in the incidence of micronuclei as compared with those in sham-exposed and in untreated control cells. However, treatment with 100 ng/ml of MMC alone resulted in a significant increase in the incidence of micronuclei. Lymphocytes from D-1, D-2, D-3, D-4, D-5, D-6 and D-7 which were pre-exposed for 20 h to RF in the S-phase of the cell cycle followed by treatment with 100 ng/ml MMC exhibited a significant reduction (43-54%) in the incidence of micronuclei as compared with those treated with MMC alone. No significant changes in the incidence of micronuclei were recorded when the cells were given the combined adaptive and challenge dose treatments (p>0.05).

Interpretation and Limitations
The results of this study indicated that cells which were exposed to an AD of RF in the G0- and G1 phases of the cell cycle did not exhibit an AR while such a response was observed when the cells were exposed in the S-phase of the cell cycle. These results confirm the results of previous studies where AR was observed in human blood lymphocytes exposed to an AD of RF in the S-phase of the cell cycle. This suggests that the timing of AD exposure of RF is important to elicit an AR. In terms of limitations, although the average SAR used in the study was 1.25 W/kg, the peak SAR of 10 W/kg could be considered ‘thermal’ and temperature increases could have had an effect on cells. The authors note however, that the temperature was well-maintained throughout the experiment. Another limitation is that since these experiments were conducted in cells, the results are difficult to extrapolate to whole-body organisms and humans.

Conclusions
This study suggests that the timing of adaptive dose exposure of RF is important in eliciting an adaptive response in human blood lymphocytes.